As is well-known, one of the purposes of the turbine exhaust case and the support rods is to support the engine's bearings and its compartment that rotatably support the engine's shafts. Typically, the turbine exhaust case comprises a pair of concentric rings and a plurality of hollow struts which support the rings to each other. The rings define the inner and outer boundaries of the engine's gas path while the struts are disposed in the gas path. The rods, supporting the bearing housing, pass through the hollow struts to interconnect the mount ring and bearing compartment. Inasmuch as the temperature of the gases in the gas path are significantly high, it is abundantly important to design these component parts to assure durability while maintaining structural integrity. Obviously, as the engine's performance and maintainability are being improved upon, it is also important that the components are easily assembled and disassembled while maintaining costs as low as possible.
Of the heretofore known bearing support mount systems for various existing engines, each exhibits disadvantages some of which are discussed hereinbelow and which are attempted to be obviated by the present invention.
For example, in many installations, the tie rods that support the bearing compartment to the outer mount ring are installed with a significantly high tension load. In order to attain these high tension loads, it is common practice to compress the outer mount ring supporting the turbine exhaust case by use of external high powered hydraulic jacks specifically designed for this purpose. When the load is applied to the outer mount ring, the rods are manually torqued down into place with the desired preload and the jacks are thereafter removed. In addition to being expensive tooling, the use of these heavy duty jacks expose the operator to some safety risks. Moreover, each rod must always be in tension and can never be in compression without incurring damage to the hardware.
In some installations, a complex double rod system is employed. Radial rods tie the bearing compartment to the outer exhaust case and tangential rods tie these radial rods from the turbine exhaust case to the outer engine case or mount ring.
In other installations, disassembly is cumbersome. For example, the outer engine casing has to be removed in order to access the nut and bolt assembly that secure these tie rods. In order to be able to remove the low pressure turbine assembly, these nuts have to be first removed. Other installations include external access to the nut and bolt assembly, but require that the entire rod be removed before the low pressure turbine module can be removed.
Obviously, in each of these instances the assembly and disassembly makes for a cumbersome and time consuming task. In a business environment where such time consuming tasks are costly, improvements in this area are much sought after.
We have found that we can obviate the problems noted above by attaching the double eye rod (radial support rod) used to support the bearing housing to the engine's mount ring by utilizing a clevis pivotally attached to the eye rod and a standard bolt attaching the clevis to the mount ring. These radial support rods in accordance with this invention can then be tightened with a standard torque wrench without the need of the hydraulic jack. By virtue of this arrangement the typical spherical bearings associated with the radial support rods are manually torqued with a standard torque wrench without the twisting and galling that has occurred heretofore, eliminating a problem that heretofore was expensive and time consuming to avoid, while at the same time eliminating the safety problems inherent in these problem solving techniques.
We have also found that by utilizing a high strength low expansion type of nickel base alloy material, such as PWA 1192, or having characteristics similar thereto, and designing the mount system such that the rods can take compressive loads under certain severe loading conditions, the torque loading at assembly can be minimized.
In a turbofan engine installation in accordance with the invention, the fan air is supplied thru the turbine engine case hollow strut thru which the rod passes to maintain rod temperatures at acceptable levels to achieve positive tension under normal operation conditions.
Installation and disassembly are facilitated by virtue of the fact that the clevis at the outside diameter of the radial rod is hinged and swings out of the mount connection. This allows for easy removal of the engine module since the rod need not be removed.
It is contemplated that within the scope of this invention that eight (8) instead of four (4) radial support rods be used so as to attain the proper stiffness and spring rate to match the spring rate of the oil film damping system utilized in a counter-rotating "piggy-back" arrangement of engine's shafts and support bearings supported in the bearing compartment.
The lubricating lines from external of the bearing compartment are arranged to pass through the hollow struts in the turbine exhaust case in such a manner so as to enhance the packaging and maintainability thereof.
The bearing housing is designed so as to be shaped in a quasi "wishbone" configuration that not only provides adequate stiffness, but provides a means to dispose the large thermal gradient between the turbine exhaust case inner ring and oil compartment in the bearing compartment.
It is also contemplated within the scope of this invention that the inner ring of the turbine exhaust case is designed to provide increased stiffness at the transition juncture where the ring attaches to the struts by way of fore and aft rings to reinforce the struts standups. This arrangement also provides increased roll and radial stiffness.
The invention contemplates the inclusion of an inspection hole in the mount ring adjacent the clevis to allow an operator access so as to be able to inspect the assembly after tensioning the rod bolts to assure proper seating.